1. Field of the Invention
The present invention relates to a creasing device for making a crease in a bundle of sheet-like members (hereinafter, referred to as “sheet(s)”) conveyed from a preceding stage before the sheet bundle is saddle-stitched and center-folded in half, and an image forming system including the creasing device and an image forming apparatus.
2. Description of the Related Art
Conventionally, so-called center-fold or center-fold binding is performed on a bundle of a plurality of sheets discharged from an image forming apparatus, in which the sheet bundle is saddle-stitched and then folded in the middle. If a sheet bundle composed of a plurality of sheets is folded at one time, an amount of stretch of a fold portion of the sheet on the outer side of the sheet bundle is larger than that of the sheet on the inner side. Consequently, at the fold portion of the outer-side sheet, a formed image area is stretched, and may result in damage on the image area, such as toner detachment. The same phenomenon occurs in other folding processes, such as Z-fold and three-fold. Furthermore, depending on the thickness of the sheet bundle, the sheet bundle may not be sufficiently folded.
To cope with such problems, there has been known a creasing device called creaser that makes a crease in a fold portion of each sheet in advance before a sheet bundle is folded in two or the like to make it easy to fold the outer-side sheet as well, and thereby prevents toner detachment. Such creasing devices include ones that make a crease in a direction perpendicular to a conveying direction by causing a roller to run, quenching with a laser, pressing a creasing blade against a sheet, or the like.
For example, the invention disclosed in patent document 1 (Japanese Patent Application Laid-open No. 2008-081258) is developed for the purpose of making a well-shaped and highly-accurate crease depending on a type of sheet; in this invention, an annular convex portion is formed on the outer circumference of a roller for making a crease and an annular concave portion is formed on the outer circumference of a roller pairing with the roller, and by causing a sheet to pass through a nip between the rollers, a crease along a sheet conveying direction is made in the sheet. It is configured that the rollers can be replaced with most preferable rollers depending on a sheet.
Furthermore, according to the invention disclosed in patent document 2 (Japanese Patent Application Laid-open No. 2009-166928), a creasing device is provided with a creasing member for making a crease extending along a predetermined line of a recording medium in the predetermined line of the recording medium, an insertion groove that is formed at a site opposed to the creasing member and into which the creasing member can be inserted, and a back-and-forth movement driving unit that drives the creasing member to move forward and backward between a standby position and a creasing position in a state where the predetermined line of the recording medium is placed between the creasing member and the insertion groove; at the time of making a crease in a direction perpendicular to a sheet conveying direction, the crease is made while reducing a pressing force applied by the creasing member, so the creasing member is moved while changing the timing to move by a plurality of individual back-and-forth movement mechanisms.
However, when a crease is made with a roller like the invention disclosed in the patent document 1, the roller is moved by a distance corresponding to the length or width of a sheet, so it takes a certain time for the roller to move, and therefore, there is a problem that it takes a longer processing time. To resolve this problem, there is a method of turning the sheet conveying direction by 90 degrees and making a crease parallel to the conveying direction in the sheet being conveyed; however, this method causes an increase in an installation area. In the case of making a crease with a laser, there is a problem that smoke or a burning smell is produced during creasing. In the case of making a crease with a creasing blade, although the crease can be easily made in a direction perpendicular to the conveying direction in a short processing time, the load is increased if an overall face of the creasing blade is simultaneously pressed against a sheet, and a larger driving force is required to make the crease.
In the invention disclosed in the patent document 2, to reduce the load, it is configured to make a crease while reducing a pressing force applied by the creasing member, and the creasing member is moved while changing the timing to move by the plurality of individual back-and-forth movement mechanisms. However, if the face of the creasing blade is brought into contact with the sheet in several batches, an uneven crease between a several-time contact portion and a one-time contact portion is made, so the creasing may not be done properly.
An object of the present invention is to shorten a time required to make a crease in a direction perpendicular to a conveying direction and reduce the load at the time of making the crease thereby improving the productivity and energy consumption.